1. Technical Field of the Invention
The present invention relates to a radiation detector, called a flat panel detector (FPD), and a radiography system with the radiation detector for medical or industrial applications.
2. Description of Related Art
A radiography system, such as a medical X-ray radiography system, requires a radiation detector. As one conventional radiation detector for acquiring radiographic fluoroscopy images of an object, there has been used a unit in which an image intensifier (referred to as an “I.I.”) is placed in a combination with an imaging tube or solid imaging element (such as a “Charge Coupled Device (CCD)”). This unit converts information about X-rays that have transmitted the object into optical signals. The optical signals are then sent to a TV camera to display them as images on a monitor or printed images on films.
However, for the above unit in which the image intensifier is combined with the imaging tube or CCD, it is fairly difficult to meet a strong demand that finer abnormalities or lesions of an object should be detected. To meet such a demand, a flat panel detector (FPD) with use of sophisticated semiconductor techniques has been developed. The FPD is a semiconductor array in which an optical-conductive layer to convert radiations into electric charges are placed over switching elements, capacitors, or others formed on for example a glass substrate. The FPD, which is relatively lighter in weight and more compact in size, has the advantage of being able to provide higher-resolution and less-distortion images.
The FPD is categorized into a direct conversion type of FPD and an indirect conversion type of FPD. The former is a device to directly convert radiations into electrical signals, while the latter is a device to convert radiations into optical signals temporarily and then to convert their optical signals into electrical signals. In the following, the direct type of FPD will be explained.
The FPD is formed into a structure in which pluralities of detection elements are two-dimensionally mapped, each of which elements converts a radiation into electric charges and stores the charges. Compared to the unit based on both of the image intensifier and the TV camera, the FPD is advantageous in various aspects. For example, the FPD is able to provide higher-resolution and less-distortion images. In addition, because the FPD outputs digital-amount signals indicative of information about a radiographic image, the image processing is easier. As being formed into a greatly thinned and light shape, the FPD can be attached to a radiography system in an easier manner. Further, the FPD is able to give a widened range of operations to a radiography system and contribute a compact and light configuration of the radiography system. When using a radiography system with the FPD as a medical imaging modality, the system gives less stress to patients, because it is formed into a light and compact appearance.
However, the FPD includes a large number of active elements made of semiconductor materials in its semiconductor array, and generates heat during the operation. Due to the generated heat, the temperature given to components (for example, the conversion membrane) influencing characteristics, such as detection sensitivity of radiations, is also changed. As a result, there is a possibility that it is difficult for the FPD to detect signals of stable images, though such stable images may be detected if no heat is generated. When the temperature of the FPD is not controlled with higher accuracy during its operation, the detection characteristic is deteriorated. Such deterioration, if occurring practically, may not lead to detection of an effectively examined image. That is, in cases where the FPD is employed by a medical imaging modality, it is no longer easy to provide clinically useful images.
For instance, it is frequent that, while fluoroscopic imaging is conducted by a cardiovascular X-ray imaging modality with such FPD, a doctor operates a catheter to inject it into a patient. In cases where the detection characteristic of the FPD begins to deteriorate during the doctor's operations of the catheter, fluoroscopic images cannot be provided any longer or only obscure images are provided. In any case, the operations to the catheter are obliged to encounter very difficult situations.
On the other hand, if the radiography system is located at a site in a lower-temperature environment, it may suffer from a lower-temperature atmosphere during its non-operation. In such a lower temperature condition, there is a possibility that the FPD is damaged. To avoid this inconvenience requires that the system be heated so as to keep its minimum temperature. Such heating means is required to work only during the rest of the system.
The FPD is structured such that the conversion membrane is deposited on the TFT array for conversion of the radiations into electric charges, in which electric charges stored at each pixel correspondingly to radiated radiations are read out by switching the TFT array. In addition, there is a difference in thermal expansion coefficient between the TFT array and the conversion membrane. If the temperature is lower, there is a fear that the conversion membrane is peeled off so that the image acquisition is stopped completely. It has also been known that if the FPD is left for a long time in a higher-temperature environment, the re-crystallization is caused in the FPD so that its service life is shortened. Thus, depending on where the radiography system is placed, some cases requires the system to be cooled down during its non-operation, as well as its heating.
As described, a region of temperatures for stabilizing the detection characteristics of the FPD during its operation is different from that required for avoiding the FPD during its non-operation from being damaged or shortened in its service life. Controlling both temperature regions of FPD by the same control means will lead to excessive control actions. For example, as to environmental temperatures, the range of 10 to 35° C. is required during the operation of the FPD, while that of 10 to 60° C. is required during the non-operation of the FPD. In such a case, if a target temperature is 30° C., there re no problems abut the temperature control during its operation. However, there is a fear that condensation may be caused during its non-operation, because there is a large difference between the environmental temperatures and the target temperature. The condensation may trigger a short circuit or electric shocks.